1. Field of the Invention
This invention relates to short range communications using surface acoustic wave (SAW) expanders and compressors.
2. Description of the Prior Art
SAW technology is well known for its excellent radio frequency (RF) performance, low cost and small size. SAW is a passive thin film technology that does not require any bias current in order to function. SAW expanders and compressors have been used in RADAR applications for many years.
The basic xe2x80x9cbuilding blockxe2x80x9d of SAW expanders and compressors is the interdigital transducer (IDT) such as shown in FIG. 1. An IDT 10 is a series of thin metal strips or xe2x80x9cfingersxe2x80x9d 12 fabricated on a suitable piezoelectric substrate 14. One set of fingers is connected to an input/output terminal 16, while the opposite set of fingers is connected to another terminal 18. In single-ended IDTs, terminal 18 is grounded. For differential input signals however, terminal 18 is a pulse input/output terminal. Spacing xe2x80x9cWxe2x80x9d between IDT segments is adjusted to conform to the desired chip period of the coded sequence. When excited by a narrow electric pulse at terminal 16, the IDT generates a coded output SAW signal that propagates in both directions perpendicular to the fingers 12. If a similarly coded SAW signal impinges on the fingers 12, then an autocorrelation function is performed and a peak, with associated side lobes, is generated at terminal 16. These abilities of SAW expanders and compressors are well known in the prior art, having been demonstrated for example in Edmonson, Campbell and Yuen, xe2x80x9cStudy of SAW Pulse Compression using 5xc3x975 Barker Codes with Quadraphase IDT Geometriesxe2x80x9d, 1988 Ultrasonics Symposium Proceedings, Vol. 1, Oct. 2-5, 1988, pp. 219-222.
Thus, the structure shown in FIG. 1 can operate as both a SAW expander, generating a SAW output from a single pulse input, and a SAW compressor, generating a single pulse or peak output from a SAW input. Terminal 16, as well as terminal 18 in differential IDTs, is both a pulse input terminal and a pulse output terminal. Conversion of an output SAW into an electrical signal for further processing in conventional communications circuits and subsequent transmission through an antenna is accomplished by adding a transmit IDT 24, aligned with the IDT 22, as shown in FIG. 2. Both IDTs can be fabricated on the same substrate 14. A SAW output from IDT 22 is converted into an electrical signal by TX IDT 24. A SAW receiver would have the same structure as in FIG. 2. A signal input to a receive IDT from receiver processing circuitry would be converted to a SAW which is input to IDT 22. Like the IDT 22, the TX IDT 24 may be a differential IDT, wherein the grounded lower terminal would be a pulse output terminal.
The geometry of adjacent IDT fingers 12 is shown in FIG. 3, where Tf is the width of a metallized finger 12 and Ts is the width of the space between the fingers 12. In typical designs both Tf and Ts are equal to a quarter of a wavelength, xcex/4. For example, for a typical SAW system operating in the Industrial, Scientific and Medical (ISM) band at 2.4 GHz the xcex/4 dimension could be in the order of 0.425 microns, depending upon the substrate chosen.
Previous SAW-based communications systems use lower frequency SAW expanders and compressors having larger and further spaced fingers in conjunction with a plurality of components such as mixers and local oscillators, as shown in FIG. 4. In the typical prior art communication system 30, a lower frequency 266 MHz signal generated by transmit IDT 20 is up-converted in mixer 34, which receives a 734 MHz signal from local oscillator 36. The resulting output from mixer 34 is filtered in high pass filter 38 to generate a 1 GHz signal for transmission through antenna 40. On the receive side, the process is reversed in antenna 42, mixer 44, low pass filter 46 and receive compressor IDT 20xe2x80x2. The TX and RX IDTs 20 and 20xe2x80x2 have the structure shown in FIG. 2. Undesirably, the mixers 34 and 44, oscillator 36 and filters 38 and 46 from the communications system 30, result in additional cost, power consumption, occupation in space and a much complex system than is desired for low-cost, low power, short range communication systems. Therefore, there remains a need in the art to reduce the number of components in such a communication system. Previously-reported designs of encoded IDT structures have employed split-electrodes of width xcex/8 within each chip segment W to suppress spurious IDT finger reflections (See, for example, M. G. Holland and L. T. Claiborne, xe2x80x9cPractical Surface Acoustic Wave Devicesxe2x80x9d, Proceedings of the IEEE, Vol. 62, pp. 582-611, May 1974). In contrast, SPUDT-type reflection gratings can be placed judiciously to enhance spurious IDT finger reflections and thereby reduce device insertion loss. These SPUDT techniques have previously been applied to realize low-loss SAW filters, where all of the IDT segments within each section W of the structure have the same polarity (See, for example, 1) C. K. Campbell and C. B. Saw, xe2x80x9cAnalysis and Design of Low-loss SAW Filters using Single-Phase Unidirectional Transducersxe2x80x9d, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. UFFC-34, pp. 357-367, May 1987; and 2) C. Campbell, Surface Acoustic Wave Devices and their Signal Processing Applications, Boston: Academic Press, 1989). These SPUDT-type enhancement techniques have not been previously applied to encoded IDTs.
High-frequency communication techniques involving more conventional non-SAW based circuits and systems also exist. BLUETOOTH(trademark) wireless technology is one such prior art example. BLUETOOTH(trademark) is a de facto standard, as well as a specification for small-form factor, low-cost, short range radio links between mobile PCs, mobile phones and other portable wireless devices. The current BLUETOOTH(trademark) short range communications specification operates in the 2.4 GHz (ISM) band; however, the BLUETOOTH(trademark) standard in its current infancy undesirably involves high cost, substantial power consumption and relatively complex hardware.
Although high frequency SAW expanders and compressors offer significant reductions in cost, power consumption, size and complexity over prior SAW-based and non-SAW based communications systems, conventional SAW expanders and compressors typically have insertion losses greater than 20 dB. This may affect the RF link budget of a communication system, as more gain would have to be designed into the system for satisfactory operation. Therefore, there remains a need for a SAW SPUDT-type expander and compressor that would improve the insertion loss and positively impact the RF link budget and complexity of the system and thereby make RF communications systems in which RF signals are generated and processed directly and solely by SAW expanders and compressors feasible.
An object of the present invention is overcome at least some of the drawbacks of the prior art.
Advantageously, the use of low loss SPUDT-type SAW devices as described in the present invention offers improved performance compared to conventional SAW devices in respect to generation of coded RF waveforms (expander) and the autocorrelation of coded RF waveforms (compressor) for communication systems.
It is therefore an object of the invention to provide a low cost SPUDT-type SAW-based communication method and system. As an illustrative example of the cost reduction resulting from the present invention, SAW devices used for filtering at near-ISM band frequencies may cost approximately $1.00 each. In contrast, a comparable semiconductor BLUETOOTH(trademark) solution may cost more than $10.00.
It is a further object of the invention to provide SPUDT-type SAW-based transmit and receive units that are easily manufactured. The manufacturing required for the present invention allows for SAW fabrication that utilizes simple, single layer photolithographic techniques.
Another object of the invention is to provide a low power SPUDT-type SAW solution for short range communications. The SPUDT uses passive thin film technology and requires only a pulse to excite and produce a coded RF waveform. Likewise it can perform an autocorrelation function passively. This compares to prior SAW techniques which require frequency conversion circuitry such as mixers, filters and oscillators, and the complex BLUETOOTH(trademark) techniques that require separate receive, transmit and processing circuitry. In mobile communication environments, power consumption and size are of primary importance.
A still further object of the invention is to provide a SPUDT-type SAW-based communication arrangement that occupies minimal space. A complete SAW package in accordance with the invention is in the order of 3 mmxc3x973 mm.
In an embodiment of the invention, a communication system comprises an expander SPUDT-type IDT which is configured to embody a code and thereby produces a coded SAW output when excited with an electric pulse, a transmit IDT positioned adjacent to the expander IDT and connected to an antenna, a receive IDT connected to the antenna, and a compressor SPUDT-type IDT, positioned adjacent to the receive IDT, which is configured to embody the code and thereby produces an electric pulse output when excited by a coded SAW input. A further aspect of the invention involves the use of a SPUDT-type IDT as the transmit IDT and the receive IDT.
Wireless communication systems according to the invention may be installed in both a wireless mobile communication device and a wireless earpiece detachable therefrom, to provide for communication between the mobile device and the earpiece. In a further embodiment of the invention, a SAW-based wireless communication system is installed in a wireless mobile communication device, a wireless earpiece detachable therefrom and a holder for the mobile device connected to a personal computer (PC), to provide for communication between the device and the PC through the holder, the device and the earpiece, and the earpiece and the PC through the holder.
The transmit IDT receives the coded SAW output from the expander IDT and produces a coded electric output signal for transmission via the antenna, and the receive IDT produces the coded SAW input to the compressor IDT from a coded electric signal received via the antenna. The electric input and output signals associated with any of the IDTs may be either unbalanced or differential signals.
In another inventive aspect, a passive wireless communication system comprises an antenna for converting received communication signals into electric antenna output signals and for converting electric antenna input signals into communication signals, a first IDT connected to the antenna and configured to produce a SAW output when a communication signal is received by the antenna, a second coded SPUDT-type IDT positioned adjacent to the first IDT and configured to produce an electric signal output when excited by the SAW output from the first IDT and to produce a coded SAW output when excited by an electric signal input, and a termination circuit connected across the terminals of the second IDT, wherein the termination circuit causes the second IDT to reflect a coded SAW output toward the first IDT in response to the SAW output produced by the first IDT, and the first IDT produces a coded electric signal as an antenna input signal in response to the reflected coded SAW output from the second IDT. The termination circuit is preferably either a short circuit connection or an open circuit.
Such a passive wireless communication system may include a SPUDT-type IDT as the first IDT. The first and second IDTs may also have differential electrical signal input and output terminals. Where the code embodied by the second IDT represents identification information or other information relating to an article with which the passive communication system is associated, the passive system may be incorporated into an identification tag, an adhesive label or an equipment nameplate. The passive wireless system preferably receives communication signals from a remote interrogation system, and through operation of the IDTs and termination circuit, automatically and passively responds to the remote interrogation system.
The code embodied by an IDT may be a Barker code such as a 5-bit or 13-bit Barker code, and may be used for example to represent identification information for an article with which the wireless communication system is associated.
According to a further aspect of the invention, a coded SPUDT-type IDT comprises a pair of substantially parallel electrically conductive rails, one or more groups of interdigital elements, each group comprising a plurality of interdigital elements, and one or more SAW reflectors. Each interdigital element is connected to one of the rails and extends substantially perpendicular thereto toward the other rail, and the particular code embodied by such an IDT is determined by a connection pattern of the interdigital elements in each group. A coded SPUDT-type compressor IDT performs a passive autocorrelation function on the coded SAW input based on the code to thereby produce the electric pulse output. A reflector in a SPUDT-type IDT may comprise a plurality of reflector grating elements, wherein the reflector grating elements of a reflector may be either open circuited or are connected to the other grating elements in the same reflector.
A SPUDT-type SAW system according to the invention may be employed in the design of virtually any new short range wireless communication system, for example to enable communication between an earpiece unit and an associated mobile communications device. The inventive systems may also replace RF signal generation circuitry in existing short range communications system, including for example BLUETOOTH(trademark) systems. A further system in accordance with the invention may be employed in xe2x80x9csmartxe2x80x9d identification tag systems and remote interrogation systems such as inventory systems and meter reading/telemetry systems.